This happens rarely, but sometimes when I'm on my platinum, it just freezes. I can't unfreeze it and I have to turn it off and I have to go from where I last saved. It can get annoying. I have a normal d.s. if it makes any difference. Since it rarely happens, I'm in no hurry for an answer, but it would be nice to have an answer.

i have that happening to my platinum on my R4 so months later i got a DSI and some other stuff. a got a M3 too, and it has platinum and it does not freeze. platinum is a old game, so that's why it probably freezes. also, if it drops on the ground, it frezzes.

This may be caused by cheats (caused by AR).
Or you could just drop it a couple times, have a faulty system, etcetera. I know when I accidentally drop mine, it usually freezes. Glad I don't do that anymore. ;DGlitches will also causes freezes. There may be more reasons, but that basically covers it.
Hope this helps!